1. Field of the Invention
The present invention generally relates to ophthalmic lenses. More particularly, the invention relates to a multifocal contact lens that includes a blended design for a segmented optical zone.
2. Description of the Related Art
Contact lenses are ophthalmic lenses worn on the anterior cornea that are widely used for correcting many different types of vision deficiencies. These include defects such as near-sightedness (myopia) and far-sightedness (hypermetropia), astigmatism, and defects in near range vision usually associated with aging (Presbyopia). A typical single vision contact lens has a real or virtual focus, which is the point at which parallel rays of light focus when the lens is placed perpendicular to the parallel rays, and an optical axis, which is an imaginary line drawn from the focus to the center of the optical zone of the lens. A posterior surface of the contact lens fits against the cornea and an opposite anterior surface has an optical zone that refracts light to correct vision. In the case of a typical spherical lens, the optical zone has a single radius of curvature; whereas the distance from any point on the optical zone to a point on the optical axis referred to as the center of curvature.
A bifocal lens has at least two optical zones, typically on the anterior surface, of the lens: a distance optical zone, for primary gaze; and a near optical zone, for down-gaze (e.g., while reading). Bifocal contact lenses are typically concentric or segmented in configuration. In a conventional, simultaneous vision, concentric bifocal contact lens, a first, centrally located, circular correction zone constitutes either distant or near vision correction, while a second annular correction zone surrounding the first zone provides the corresponding near or distance vision correction, respectively. In a conventional, alternating vision, bifocal contact lens of the segmented or translating type, the lens is divided into two optical zones. Usually, the upper zone is for distance vision correction whereas the lower zone is for near vision correction. With such a translating lens, the distance portion (upper zone) of the lens is predominately in front of the pupil of the eye in primary gaze, while in down-gaze, the add power or near portion (lower zone) of the lens is predominately aligned to the pupil. Effective use of an alternating vision bifocal contact lens requires translation of the optical zones across the pupil when the eye changes from primary gaze to a downward gaze. In such a situation, the lens must move such that the pupil is predominately subtended by the distance zone for primary gaze and predominately subtended by the near zone for down-gaze. The design of a segmented optical zone contact lens requires surface blending between the distance portions such that vision is not degraded as the contact lens translates from one optical zone to another across the pupil. Specifically, in a bifocal, segmented optical zone contact lens, the images, on the retina, from both an upper and lower optical zones must be laterally coincident to minimize or eliminate ghost images. Ghost images are caused when the images from multiple optical zones have lateral separation. Additionally, the blend zone between the upper and lower optical zones can create a variable power optical zone with lateral image separation that likewise causes ghost images to the wearer.
Therefore, it would be advantageous to provide a bifocal or multifocal contact lens characterized by having minimal or no ghost images from segmented optical zones. It is accordingly to such an ophthalmic lens, and the design aspects that this design configuration permits, that the present invention is primarily directed.